1. Field of the Invention
This invention relates to a wood chip recovery system which removes bark from roundwood or wood chips. It is desirable for such structures of this type to recover the wood chips from the bark without introducing contaminants, such as metal ions like iron, into the debarked roundwood or wood chips.
2. Description of the Related Art
Availability of wood from the forest is declining because of an increase in demand from new users and due to added limitations on harvesting. These factors make it imperative that ways are identified to improve wood recovery in order to ensure a sufficient, future and economical wood supply.
Harvesting trees as whole-tree chips, or WTC, is a commercial approach proven to increase wood recovery. WTC processing increases wood recovery by around twenty-five percent beyond recovery of commercial-grown wood, which recovery does not include branches and tree-top wood chip material. However, the high bark content of WTC, which can approach twenty percent, has been an impediment to submitting to a WTC utilization in applications like wood pulping and paper making.
High bark content in chips is undesirable because bark carries non-process elements (NPE) like metal ions that are detrimental to the paper pulp processing by reducing pulp yield and bleaching effectiveness while increasing scaling of heated process surfaces and corrosion. Sand from bark also accelerates erosion of valves, like those in pulp digesters, piping and elbows.
It is known, in wood chip debarking systems, to employ the use of ball-milling which acts as an abrasion means to remove the bark from the logs or chips of logs. Exemplary of such prior art bark removal systems is U.S. Pat. No. 4,332,353 ('353) to H. L. Lario et al. entitled "Procedure For Mechanically Raising Wood Content In Wood Chips". While the '353 reference discloses the removal of bark and green stuff matter from the wood chips by grinding, the grinding is accomplished through the use of ball-milling.
It is well known that using an iron attrition means, such as ball-milling, introduces iron by particles, dust, and/or discrete spalled pieces from ball/ball and ball/drum impacts. As a result of both bark grinding and metal particles, the ball-milling can therefore yield black chips difficult to purify. Also, the metal from the ball-milling can participate in sensitive chemical reactions in paper pulping, bleaching, recovery and fuel burning systems, and add high metal ions which are undesirable in most paper products.
It is apparent from the above that there exists a need in the art for a roundwood or wood chip system which is capable of debarking the logs or chips, but which at the same time avoids the introduction of the contaminants like NPE into the debarked wood chips. It is the purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.